Ferroelectric liquid crystals and antiferroelectric liquid crystals are generally known liquid crystals that exhibit smectic phases. These liquid crystals are used in image-producing displays by utilizing the properties that both types of liquid crystals possess spontaneous polarization, and that the direction of the spontaneous polarization changes under the influence of an external electric field. Liquid crystal panels using antiferroelectric liquid crystals, for example, have been researched vigorously since it was reported in Japanese Patent Unexamined Publication No. 2-173724 by Nippondenso and Showa Shell Sekiyu that such liquid crystal panels provided wide viewing angles, were capable of fast response, and had good multiplexing characteristics.
However, when a still image or the like is displayed on an antiferroelectric liquid crystal display with the same image pattern staying on the display for a long period of time, a phenomenon occurs in which when a different image pattern is displayed on the screen, the previously displayed image remains visible on the screen (this phenomenon is hereinafter called the "burn-in phenomenon"). This burn-in phenomenon is believed to be caused by the fact that antiferroelectric liquid crystals have a layer structure.
It is also known that, in ferroelectric liquid crystal displays using ferroelectric liquid crystals, a similar burn-in phenomenon occurs when the same pattern such as a still image has been displayed on the screen for a long period of time. This burn-in, unlike the case of antiferroelectric liquid crystals, is believed to be caused by the movement of impurity ions contained in the liquid crystal cell. Ferroelectric liquid crystals have a spontaneous polarization, and when an external voltage is 0 V, an internal electric field due to the spontaneous polarization is always present in a direction perpendicular to the liquid crystal cell. Impurity ions in the liquid crystal cell move toward the cell-substrate interface in such a manner as to cancel the internal electric field, and an ion electric field due to the impurity ions occurs in a direction opposite to the direction of the internal electric field. If this condition continues for a long period of time, the impurity ions are adsorbed onto the cell-substrate interface. As a result, the ion electric field persists after the internal electric field due to the spontaneous polarization is removed. This is believed to cause the burn-in phenomenon.